Bad Air Days in Hamilton County, Ohio: 9 a Year
9 bad air days a year
Among the worst tenth of monitored US counties. Ranked 94 of 996 monitored counties, where 1 is worst.
What makes them bad: mostly ozone: summer smog (16% particle days).
Which months to watch
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in June and July and August. Averaged over 2016 to 2025.
Is a purifier worth it here?
- Ozone drives many bad days here. Purifiers and filters do not remove ozone, so do not buy one for that. Ozone peaks on hot afternoons: exercise outside in the morning.
Better or worse than it was
In 2000 to 2009 this county averaged 37 bad days a year; in 2016 to 2025, 9. The air has improved. The worst recent year was 2023, with 19 bad days on 365 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2023-06-28 | 193 | PM2.5 |
| 2018-07-14 | 159 | Ozone |
| 2023-06-27 | 157 | PM2.5 |
| 2025-07-05 | 154 | PM2.5 |
| 2022-06-29 | 151 | Ozone |
An ordinary day in Hamilton
Across 3,653 monitored days from 2016 to 2025, the air in Hamilton County rated Good on 33% of them, less often than the 72% typical of monitored counties, and Moderate on 65%. The median day sat at AQI 55; one day in ten went above 74.
The pollutant setting the AQI on an ordinary day here:
- fine particles (PM2.5)74%2,706 days
- ozone24%889 days
- nitrogen dioxide2%58 days
Hamilton County by year
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 365 | 125 | 227 | 13 | 1 | 54 | 154 |
| 2024 | 366 | 164 | 191 | 11 | 0 | 52 | 140 |
| 2023 | 365 | 129 | 217 | 19 | 2 | 54 | 193 |
| 2022 | 365 | 125 | 236 | 4 | 1 | 54 | 151 |
| 2021 | 365 | 109 | 251 | 5 | 0 | 56 | 140 |
| 2020 | 366 | 105 | 254 | 7 | 0 | 55 | 126 |
| 2019 | 365 | 99 | 260 | 6 | 0 | 57 | 119 |
| 2018 | 365 | 97 | 260 | 8 | 1 | 57 | 159 |
| 2017 | 365 | 118 | 242 | 5 | 0 | 55 | 129 |
| 2016 | 366 | 118 | 236 | 12 | 0 | 55 | 140 |
| 2015 | 365 | 119 | 240 | 6 | 1 | 55 | 151 |
| 2014 | 365 | 62 | 292 | 11 | 0 | 61 | 143 |
| 2013 | 365 | 65 | 296 | 4 | 0 | 61 | 112 |
| 2012 | 366 | 46 | 293 | 27 | 5 | 61 | 182 |
| 2011 | 365 | 104 | 230 | 31 | 5 | 59 | 179 |
| 2010 | 365 | 23 | 311 | 31 | 3 | 71 | 172 |
*2025 may be a partial year in EPA's file.
Elsewhere in Ohio
- Cuyahoga County7/yr
- Lucas County7/yr
- Lake County7/yr
- Butler County6/yr
- Warren County5/yr
- Geauga County5/yr
- Ashtabula County4/yr
- Franklin County4/yr
How this is counted
A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day, and on why a purifier does not touch ozone in smoke vs ozone.